The present invention relates to a slip-controlled brake system for automotive vehicles with a deceleration-sensitively actuated brake pressure modulator comprising a shaft rotating at the angular velocity of the controlled vehicle wheel or at a multiple of that velocity and an inert mass arranged on the shaft and rotating together with the shaft. The inert mass is axially displaceable in opposition to a return force on the shaft when the controlled vehicle wheel is decelerated and a wheel deceleration threshold is exceeded and which thereby acts upon a control element controlling the brake pressure modulation. The system further includes a clutch and brake mechanism which permits a difference in speed between the shaft and the inert mass when the vehicle wheel is decelerated, and which causes the axial displacement as well as a deceleration of the inert mass during the control phase.
A brake system of this type is known (SAE Technical Paper Series, No. 840464, Newton, Riddy, Intern. Congress, from Feb. 27, until Mar. 2, 1984). The inert mass of the modulator, which inert mass is shaped in the form of a flywheel, is connected with the rotating shaft which is driven by the vehicle wheel through a ball and ramp arrangement. If a predetermined wheel deceleration threshold is exceeded, the axial movement of the flywheel on the shaft causes the opening of a valve through the lever of a control element by which means the brake pressure in the wheel brake is relieved and a re-acceleration of the wheel is brought about. In addition, the ball and ramp arrangement comprises a brake mechanism which, after the displacement of the flywheel and the opening of the pressure relief valve, has a braking effect on the flywheel which, through its inertia, rotates faster than the shaft during this phase, and which brake mechanism thus causes a deceleration of said flywheel. The force determining the deceleration is derived from a return spring abutting on a peripheral edge of the shaft. The deceleration force is, therefore, constant.
In controlled brake systems of this type, the speed reference quantity for the controlled wheel required for slip monitoring and slip control is obtained from the flywheel and the mechanism described above. In this case, the speed reduction of the inert mass follows from the illustrated brake system which effect is determined by the return spring and by friction elements within the clutch. The difficulties encountered with the formation of a speed reference quantity on the basis of one wheel only follow from that during the phase with an instable rotational behavior of the wheel, that is, as long as the vehicle wheel is in an excessive slip phase, the reference quantity cannot be derived from the wheel speed as such or from other wheels but must be reduced with a predetermined value which is independent of the actual rotational behavior of the wheel. In the mechanical system described, this is achieved through a deceleration of the flywheel by means of friction linings and the return spring. In electronic systems, however, information about the vehicle speed and deceleration required to form the reference quantity can be gathered during this phase by means of measurements on the other wheels which run in a stable manner.
Ideally, the speed reference quantity should decrease analogously to the vehicle deceleration during a phase of instability of a vehicle wheel. When each wheel is controlled individually and the reference quantity is generated individually, however, no information about the vehicle deceleration is available during the slip phase. For safety reasons, the decrease in reference quantity must, during this control phase, therefore be regulated such that even in the event of stops on road surfaces with high friction coefficients. That is, on dry non-skidding road surfaces, the speed reference quantity will not, upon completion of the critical wheel slip phase, assume a higher speed value than the wheel running in a stable manner since, otherwise, a critical wheel slip would be simulated and the brake pressure would be reduced during this stable phase. In situations with lower friction cooefficients, that is on slippery road surfaces, however, the course of the reference quantity applicable to road surfaces with high friction coefficients, to which the system is set, is not optimal. In the event of a slippery road surface on which the maximum possible vehicle deceleration is relatively low, the reference quantity wil be reduced too fast, which will result in an early termination of the pressure reduction and thus in an unfavorable control behavior.
It is, therefore, an object of the present invention to overcome these difficulties and to create a brake system of the type initially referred to, wherein the vehicle reference speed, in particular the rate of decrease of this reference quantity during a phase of instability, conforms to the instantaneous friction coefficient, that is, to the respective road surface.